Electroluminescent lamp structures



Jan. 31, 1956 ELECTROLUMINESCENT LAMP STRUCTURES Filed Oct; 7, 1952 I NVENTOR Joseph L. Gillson, J.

ATTORNEY J. L. GILLSON, JR 2,733,367

Patented Jan. 31, 1956 2,733,367 ELECTRQLUMINESCENT LAMP STRUCTURESJoseph L. Gillson, Jr., Wilmington, DeL, assignor to E. I. du Pont deNemours and Company, Wilmington, Del., a corporation of DelawareApplication October 7, 1952, Serial No. 313,526 1 Claim. (Cl. 313-108)This invention relates to structures for luminescence and, moreparticularly, to improved structures forelectroluminescence.

due provision bemg made for transmission of the light emitted by thephosphor. In the electroluminescent lamp structure disclosed in U. S. P.2,566,349, for exlight-transmitting electrically-conductive layer and anelectrically-conductive layer which may, but need not be,light-transmitting. The two electrically-conductive layers are connectedto a source of alternating or pulsating current of the desired potentialwhich serves to excite the phosphor material to luminescence.troluminescent lamp differs from the fluorescent lamp in that in thelatter, the voltage or field is placed across a gas and the radiationfrom the gas is used to excite a phosphor; whereas, in theelectroluminescent lamp, light is obtained by the direct application ofa varying voltage across a phosphor or by placing the phosphor in avarying electric field.

In the operation and construction of electroluminescent lamp structures,it is known that a number of factors affect the intensity of the lightemitted. Thus, the

thickness, the resistivity, and the dielectnc constant of luminosity maybe nited States Pa -5C 2 luminescent structures in a great variety oflighting applications. Other objects will be apparent from the following description of the invention.

I have found that theunique combination of chemia semi-transparent andflexible supelectrolumi- Polyethylene terephthalate may be prepared bythe condensation of ethylene glycol and terephthalic acid, or,preferably, by carrying out an ester group containing from 1 to 7 carbonatoms. Owing to the availability of dimethyl terephthalate, it ispreferred to carry out the ester interchange reaction between ethyleneglycol and dimethyl terephthalate. Polyethylene terephthalate andanalogous polymers are described.

terephthalic acid, the alkyl The invention will now be specificallydescribed with reference to the accompanying drawing wherein! Figure 1is a cross-sectional view of a preferred elecate film, each with thephosphor-bearing element thin coatings or films 4 and 5 ofelectrically-conductive material which provide the lamp electrodes andwhich are connected through lines 6 and 7 to a suitable source (notshown) of alternating or pulsating electric current. At least one ofthese coatings should be of a nature and thickness such as would notsubstantially impair the light-transmitting qualities of the base filmupon which it is formed.

Either unstretched or stretched polyethylene terephthalate film may beused for purposes of this invention. However, inasmuch as the pertinentphysical properties, e. g.,

impact strength, flex life, etc., of stretched film which has beenstretched to substantially the same extent. 1. e., from 2.5 to 3.25times (X) its original dimensions, in both directions, and heat-setunder tension at a temperature within the range of 150 to 250 C., it ispreferred to use suchfilm in the electroluminescent lamp structures ofthis invention. The-combination of chemicai, physical and electricalproperties of biaxially oriented, balanced polyethylene terephthalatefilm, together with the production of such film, is fully disclosed incopending application U. S. Serial No. 287,345 filed May 12, 1952, inthe name of B. W. Fuller. With respect to the use of polyethyleneterephthalate film as a dielectric member in electroluminescentstructures, its high dielectric strength (high voltage breakdown),dielectric constant, high resistivity,, moisture insensitivity, and thefact that the film may beproduced in very thin gauges substantially freefrom electrical faults, are a number of the essential properties whichmake it outstanding in the application of the present invention. Table 1represents a comparison of a number of these properties with the sameproperties of various other dielectric materials.

TABLE 1 Poly. Tcre.

Film 0.002" in thickness Property (stretched 3X Mica Nylon biaxially andheat-set at 20 C.) Tensile Strength (lbs/sq. .in.)- 24,000 5,000 9,000Percent Tensile Strength Loss 7 I Alter 24 Hours at 200 C 1 l 1 94 20Dielectric Strength (Volts/0.001") 4, 500 3 3, 000 3, 000 DielectricFatigue (Voltsl0.001). 200-300 3 300 Moisture Absorption (24 Hours inWater) (Percent by weight). 0. 1 24 2 Chemical Resistance Good Good Fair1 Fish paper/mica laminate .012 in. thick with approx. .002 in. of fishpaper.

0.001 Polyethylene terephthalate film (stretched 3X biaxially andheat-set at 200 C.)

5 Pure mice .002 in. thick.

The following tables, Tables 2 and 3, illustrate certain of theseelectrical properties of biaxially oriented, polyethylene terephthalatefilm under various conditions of temperature and humidity. Table 2 showsmeasurements of dielectric strength at temperatures from 0 to 150 C. at60 cycles per second. The average dielectric strengths (breakdownvoltages), on the basis of ten samples, range from 3,150 to 4,500 voltsper 0.001; and this indicates that this property is outstanding. Mostdielectric materials presently in use have a breakdown voltage in therange of 500 to 1,000 volts per 0.001.

TABLE 2 Dielectric strength of polyethylene terephthalate film 1 in airAverage Temperature, C. Kilovolts Gauge Volts/0.001

(Inches) 1 Stretched 3X biaxially and heat-set at 150 C.

Table 3 shows measurements of volume resistivity at temperatures from 0C. to 150 C.

TABLE 3 Volume resistivity of polyethylene terephthalate film 1Temperature, C.: Volume Resistivity (ohm cm.)

1 Stretched 3X biaxially and heat-set at 150 0.

The following table (Table 4) illustrates the surface resistivity of thefilm at 25 C. over the 0-l00% relative humidity range at 500 volts,direct current.

TABLE 4 Surface resistivity of polyethylene zerephthalate film 1 at 25C. Relative humidity, percenti Surface resistivity (ohms) 12 100 4.8 l0

1 Stretched ax blaxially and heat-set at 150 0.

Table 5 clearly illustrates the outstanding improvement in most of theimportant physical properties by stretching the amorphous filmbiaxially. In actuality, if such pertinent physical properties astensile strength, impact strength, fiex life, etc., of polyethyleneterephthalate film could not be improved by stretching, the utility ofthe film as a dielectric material would be greatly curtailed.

TABLE 5 Polyethylene terephthalate film Stretched Stretched StretchedProperty 3X Bi- 2.5x Bi- 2X Binxinlly axially axially Thickness (inches)0. 001 0. 001 0. 001 Tensile (p. s.. 26,000 10, 500 16,000 BreakElongation 100 160 200 Impact (kg-em.) 76 63 40 Tear Strength (g 22 1020 Flex Life (cycles). 20, 000 Water Vapor Permeabllityf g./100 sq.meters/hr 160 Tensile Modulus (p.s. 1.) 500, 000 105, 000 470, 000Density, g./ce 1 1 At slow elongation rate.

The foregoing tables of pertinent electrical properties of polyethyleneterephthalate film illustrate outstanding advantages of employing thisfilm as the dielectric in electroluminescent structures, andparticularly outstanding is the high breakdown voltage of the film atvarious temperatures. In essence, this indicates that higher potentialsmay be applied to excite the phosphors and, consequently, light ofhigher intensity is emitted. Moreover, because of its excellent strengthproperties and because it may be prepared substantially free ofelectrical faults-in thin sections, 'biaxially oriented, balancedpolyethylene terephthalate film may be employed in thick nesses as-thinas-0;000l"'0.00025; and films up to 0.01 are useful in specificstructures of this invention. In such thin sections, the film is highlyflexible and may be employed to fabricate electroluminescent structuresin roll form, e. g., to be applied to walls, ceiling, etc., or drapedaround objects to produce decorative illumination. Possiblestructures-of this type will be described hereinafter.

Where polyethylene terephthalate film is to serve as thephosphor-containing medium, any convenient expedient may be employed toembed or to disperse the phosphor particles in the film. For example,suitable electrical field-responsive phosphors, e. g., an activated aises-ea directlyin molten polyethylene terephthalate, and thereafterthemolten material may be extruded directly into film form. Furthermore,depending upon the general physical requirements of the overallelectroluminescent structure, the film containing the dispersedphosphors may or may not be subjected ,to biaxial stretching followed byheat-setting in order to obtain a film having physical, electrical andchemical properties hereinabove recited.

In addition to its outstanding electrical, chemical and physicalproperties, polyethylene terephthalate filmv is readily metallized byknown techniques and, hence, is particularly suited for use as theelectrode-supporting, light-transmitting elements of electroluminescentlamp structures. Metals such as aluminum, zinc, silver, etc., may bereadily applied to one side of the film by evaporation at high vacuumaccording to techniques well known in the art, to form thereoncontinuous metal coatings which, though electrically conductive, may beso thin, e. g., in the neighborhood of 0.0001 mil, as not tosubstantially impair the light-transmitting properties of the film. Onthe other hand, instead of having a thin continuous coating of metal onthe film, a printed pattern of a metal deposit may be employed to give adecorative efi'ect. That is, light of highest intensity would be emittedfrom areas immediately adjacent to the edges of the metallized portions.Alternative methods of rendering polyethylene terephthalate filmelectrically conductive include applying coatings of conductive sizescomprising various inorganic salts such as aqueous solutions containingzinc chloride, lithium chloride and other conductive materials whichwould not substantially impair the light-transmitting properties of thefilm. Other types of conductive coatings which might be employed includevarious polyelectrolytes, such as polyacids or poly-quaternary salts.Still another method of rendering the film conductive consists indissolving various inorganic salts in molten polyethylene terephthalate,forming homogeneous solutions thereof, and thereafter extruding themolten polymer into film form. If desired, the phosphor-bearing elementand at least one electrode of the lamp structure may be combined simplyby applying metal, as above indicated, to phosphorbearing polyethyleneterephthalate film.

The following example is illustrative of a preferred embodiment whereineach of the elements of the representative electroluminescent lampstructure shown in Figure 1 comprises polyethylene terephthalate film.

EXAMPLE 1 A specially prepared phosphor comprising zinc sulfide withcopper and lead particles as activators was uniformly dispersed inmolten polyethylene terephthalate at a temperature of 300 C. At thistemperature, the molten polymer was extruded into the form of a filmonto the surface of a rotating quench drum maintained at a temperatureof 6080 C. The resulting film was 0.002 in thickness, and this film wasemployed as element 1 of the electroluminescent lamp structureillustrated in Figure l. Element 2 was a polyethylene terephthalate film0.001" in thickness and stretched 3 times (3X) in both directions andheat-set at 200 C. Element 4 was an adherent coating of aluminum (0.0025mil in thickness) on the polyethylene terephthalate film. Element 3 wasthe same type of film as that of element 2. Element 5 was also anadherent aluminum coating which was somewhat thicker (0.02 mil inthickness) than element 4 in order to provide a reflective backing forlight emitted through elements 2 and 4. These elements were bondedtogether with an adhesive cement comprised of neoprene andphenol-formaldehyde resin in toluene. A source of alternating current ata frequency of kilocycles was employed to apply a potential of 400 voltsbetween the electrodes of the ca- 6 paeitor construction. 5 a] result, agreen glow was emitted 'through elements} and'4. I

From the foregoing, itis readily-apparent that inorder to take fulladvantage of the outstanding combination of physical, chemical andelectrical properties offered by bi axially oriented, heat-setpolyethylene terephthalate film in electroluminescent structures, theentire structure should be made up of components fabricated fromoriented, heatset film. In essence, the resulting-structure forelectroluminescence consists of individual films, i. e., biaxiallyoriented and heat-set, having substantially equivalent properties; and,consequently, the compositeof thesefilms is outstanding with respect toits combination of physical chemical and electrical. properties.However, polyethylene terephthalate film may also be used to greatadvantage in conjunction with conventional materials of constructionemployed heretofore in electroluminescent structures. For example, aconductive glass may be made by exposing a heated glass surface tovapors of silicon, tin, or titanium chloride, and placing the freshcoating in a reducing atmosphere. This coated glass may be used as thelighttransmitting and electrode elements 2 and4 in electroluminescentstructure of Figure l. A selected phosphor, for example, a powderedmixture of zinc sulfide and zinc oxide, which has been activated atelevated temperatures in an inert gas, may be imbedded in suchdielectric materials as suitable solidified oil, wax, plasticizedcellulose nitrate, etc., this forming the phosphor-containing element 1.Further, the conducting layers 4 and 5 may be sheets of metal foilinstead of being in the form of an adherent coating.

Further advantage of the high dielectric strength of polyethyleneterephthalate film may be taken by employing the film as a dielectricprotector. In other words, in cases where the dielectric strength of thephosphor-containing layer is considerably lower than that ofpolyethylene terephthalate film, the phosphor-containing layer may beinserted between adjacent layers of polyethylene terephthalate film; andthe conductive layer is on the outside (see Figures 2 and 3), e. g., ametal-coated polyethylene terephthalate film in which the film isimmediately adjacent the phosphor-containing layer and the metal coatingis on the outside. The following examples illustrate the use ofpolyethylene terephthalate film in this capacity in addition toillustrating its use in conjunction with conventional materials ofconstruction employed heretofore in electroluminescent structures.

EXAMPLE 2 Referring to Figure 2, an electroluminescent structure wasfabricated by employing a polyethylene terephthalate film (element 9),i. e., 0.001 in thickness, and stretched 3 times (3X) in both directionsand heat-set under tension at 200 C., coated on the outside with a layerof aluminum (element 10) 0.0025 mil in thickness. Element 8 of thestructure was a thin film of beeswax (0.0005" in thickness) in which wasdispersed a specially prepared phosphor comprising zinc sulfide withcopper and lead as activators. Element 11 was a solid brass plate about0.125 in thickness. A source of alternating current at a frequency of 10kilocycles was employed to apply a potential of 400 volts between theelectrodes of the capacitor construction. As a result, a green glow wasemitted through the thin coating of aluminum on the polyethyleneterephthalate film.

EXAMPLE 3 A structure corresponding to the arrangement of Figure 3 wasfabricated and element 12 was a thin film (0.0005 in thickness) ofbeeswax containing a dispersion of the phosphor of Example 2. Thisphosphor-containing dielectric was protected with two adjacent films ofpolyethylene terephthalate (elements 13 and 14). These films had acoating of aluminum (0.0025 mil) on the outer surfaces, and the aluminumcoatings were elements 15 and 16. A source of alternating current at afrequency of 10 7 kilocycles was employed to apply a potential of 400volts across the electrodes. In order that the illumination could beobserved, the aluminum coating (element 15) on the polyethyleneterephthalate film was removed from the edge portions of three sides,and a green illumination was emitted from those portions of thestructure. The light of highest intensity Was emitting from portionsimmediately adjacent to the edges of the metallized area.

As many widely difierent embodiments may be made without departing fromthe spirit and scope of this invention, it is to be understood that saidinvention is in no wise restricted save as set forth in the appendedclaim.

I claim:

A flexible electroluminescent lamp structure which comprises a'fiexiblebottom layer of biaxially oriented, balanced, heat-set polyethyleneterephthalate film having a fiexible coating of metal, a flexibleintermediate layer of polyethylene terephthalate film containing aphosphor material which is excited to luminescence under the influenceof an electrical field, and a flexible top light-transmitting layer ofbiaxially oriented, balanced, heat-set polyethylene terephthalate filmhaving a coating of metal about 0.0001 of a mil in thickness.

Mager Sept. 4, 1951 Mager Jan. 6, 1953

